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What Is Surface Tension?

PhysicsChemistry

Quick Answer

Surface tension is the elastic-like force at the surface of a liquid, caused by cohesive forces between molecules pulling surface molecules inward, making the surface behave like a stretched membrane.

The Full Story

Molecules inside a liquid are pulled equally in all directions by their neighbours, but molecules at the surface have no neighbours above them, so they experience a net inward pull. This creates a "skin" effect — the surface minimises its area, which is why small volumes of liquid form spheres (the shape with the smallest surface area for a given volume). Water has unusually high surface tension (72.8 mN/m at 20 degrees Celsius) due to hydrogen bonding between molecules. This allows insects like water striders to walk on water — their hydrophobic leg hairs distribute their weight over the surface without breaking through. Surfactants (soaps and detergents) work by reducing surface tension, allowing water to spread and penetrate fabrics or wash away grease.

Key Facts

1.Water surface tension at 20 degrees Celsius is 72.8 millinewtons per metre — among the highest of any common liquid, exceeded among room-temperature liquids only by mercury (485.5 mN/m).
2.A water strider can support about 15 times its body weight on water surface tension — its legs make tiny dimples in the surface without breaking through.
3.Soap reduces water surface tension from about 72 mN/m to roughly 25-40 mN/m, which is why soapy water spreads and wets surfaces more easily than pure water.

YouTube Angle

Suggested video title for this topic:

"Surface Tension — Why Water Acts Like It Has a Skin"